Acoustic-Wave-Induced Ferromagnetic-Resonance-Assisted Spin-Torque Switching of Perpendicular Magnetic Tunnel Junctions with Anisotropy Variation

Abstract

We have investigated Surface Acoustic Wave (SAW) induced ferromagnetic resonance (FMR) assisted Spin Transfer Torque (STT) switching of perpendicular MTJ (p-MTJ) with inhomogeneities using micromagnetic simulations that include the effect of thermal noise. With suitable frequency excitation, the SAW can induce ferromagnetic resonance in magnetostrictive materials, and the magnetization can precesses in a cone with high deflection from the perpendicular direction. With incorporation of inhomogeneity via lateral anisotropy variation as well as room temperature thermal noise, the magnetization precession in different gains can be significantly incoherent. Interestingly, the precession in different grains are found to be in phase, even though the precession amplitude (angle of deflection from the perpendicular direction) vary across grains of different anisotropy. Nevertheless, the high mean deflection angle can complement the STT switching by reducing the STT current significantly; even though the applied stress induced change in anisotropy is much lower than the total anisotropy barrier. This work indicates that SAW assisted switching can improve energy efficiency while being scalable to very small dimensions, which is technologically important for STT-RAM and elucidates the physical mechanism for the potential robustness of this paradigm in realistic scenarios with thermal noise and material inhomogeneity.

Magnetic state switching in FeGa microstructures

“…The zoom-in image of Figure 4 (e) shows the microscopic random in-plane orientations of the [001] easy axis, where the magnetic moments within each grain were assigned a unique MCA orientation associated with the grain orientation. (32) The formation of the multi-domain structures in the modelling approach included a coupling response from the long range (> 1 um) interactions of the shape anisotropy, and short range intergranular interactions (GS ~ 40 nm) of the MCA. The 2 nd order modulation and the 4 th order proportionality are necessary to model the multi-domains seen in Figure 3 (a-b).…”

Energy Harvest in Ferromagnet‐Embedded Surface Acoustic Wave Devices

“…[ 3 ] In addition to the valuable practical applications, SAW also provides a versatile platform for studying fundamental physics. So far, the interactions between SAW and ferromagnetic (FM) materials have been extensively investigated to achieve acoustic control of magnetism, [ 4 ] including magnetoelastic interactions, [ 5,6 ] SAW‐driven ferromagnetic resonance, [ 7,8 ] SAW‐induced magnetization switching, [ 9–11 ] SAW‐assisted spin‐transfer torque, [ 12–14 ] and spin–orbit torque [ 15 ] switching, SAW‐controlled domain wall motion, [ 16,17 ] and skyrmion creation. [ 18 ] Meanwhile, SAW can couple with the spin angular momentum of electrons to generate spin current by acoustic spin pumping in FM metals with magnetoelastic coupling, [ 19–21 ] spin rotation coupling in light metals [ 22,23 ] or acoustic spin Hall effect in metals with strong spin–orbit coupling.…”

“…[3] In addition to the valuable practical applications, SAW also provides a versatile platform for studying fundamental physics. So far, the interactions between SAW and ferromagnetic (FM) materials have been extensively investigated to achieve acoustic control of magnetism, [4] including magnetoelastic interactions, [5,6] SAWdriven ferromagnetic resonance, [7,8] SAW-induced magnetization switching, [9][10][11] SAW-assisted spin-transfer torque, [12][13][14] and spin-orbit torque [15] switching, SAW-controlled domain of anomalous Nernst signal. To obtain maximum anomalous Nernst electric field E ANE , we pattern the film into a Hall bar configuration to detect the voltage drop perpendicular to the wave vector k of the SAW, namely, in the transverse direction, satisfying M × k. By scanning the out-of-plane magnetic field, hysteresis loops appear in both transverse and longitudinal voltages, which are standard ANE curves.…”

Surface acoustic wave induced modulation of tunneling magnetoresistance in magnetic tunnel junctions

“…SAW based control of magnetization was also demonstrated in dilute semiconductors such as GaMnAs [24][25]. SAW of few MHz frequency can quasi-statically reduce the energy barrier within a nanomagnet to assist spin transfer torque-based switching [26] while SAW of few GHz can be used to excite acoustically driven ferromagnetic resonance in nanomagnets [27][28][29]. SAW of ~100 MHz frequency has been used to actuate extreme sub-wavelength magneto-elastic antennas consisting of magnetostrictive nanomagnets delineated on a piezoelectric substrate [30].…”